Multimedia applications are amongst the fastest growing applications in the computer industry today. These applications typically integrate different types of data, for example text, audio, video and graphics information. In order to effectively create multimedia applications, authors rely heavily upon multimedia authoring tools. Video on demand and rich interactive multimedia titles have large amounts of digital video and audio, which depend on a continuous (isochronous) output stream. Currently, no multimedia authoring tools support automatic network delivery of titles, but rather assume that all of the title content is resident on the local PC, typically on hard disk or CDROM. In today's systems, multimedia authoring tools typically load performance critical sections of the title into memory. This approach will not work with digital video and audio because a typical high quality 320.times.240 resolution digital video clip consumes 500 Kb to 1 Mb of data per second. Clearly, 30 second video clips cannot be stored in memory or on the local hard disk.
One prior computer system allows for retrieval of audio and video data from a remote computer using a separate programming interface. The programming interface is then used for traditional remote access. Retrieval of audio and video data through this interface, however, requires the use of -special commands and instructions. These special commands and instruction increase the burden on software developers trying to develop multimedia software.
Audio and video data, by their very nature, are also time dependent. Current multimedia authoring tools assume that the bandwidth characteristics of the network are fixed and can be determined reliably without delay. This assumption is inaccurate, however, because network bandwidth is generally variable over time, and subject to latency (time delay between when a request for data is made and the data actually arrives at the requester). The audio and video data must be retrieved at the appropriate time and must be available for use by the computer system at the appropriate time. When data is retrieved over a network from a remote location, however, the arrival of the data at the computer system is often unpredictable and causes the data to be temporarily unavailable. The unavailability of data can be caused by network collisions and bandwidth differences between different segments of the network. The fast-growing "Information Superhighway" also changes the dynamics of multimedia delivery systems. Increasingly, multimedia servers will be connected, via cable, telephone or other communications lines, to network connections that are capable of more than 10 Mbits per second downstream bandwidth. This increase in capability does not, however, resolve the problem of variable network bandwidth and latency.
As a result of this uncertainty, the time dependent audio or video presentation may experience a discontinuity due to the unavailability of audio or video data. In the case of video data, there is typically a sequence of images (frames) which, when the images are displayed in rapid sequence (e.g., each frame being displayed for 1/30th of a second immediately after displaying a prior frame), will create the impression of a motion picture. The human eye is particularly sensitive to a discontinuity in a motion picture and the discontinuity would manifest itself as a stutter in the video image or a `freeze-up` (e.g., a single frame being displayed considerably longer than 1/30th of a second) in the video image. With audio presentations, the discontinuity would manifest itself as a period of silence or a pause, or perhaps a change in the pitch of the audio sounds. These types of discontinuities create undesirable effects in multimedia presentations and can noticeable degrade the quality of a presentation when audio or video data is required from a remote location over a network.